An SRAM has a larger number of elements constituting a memory cell compared, for example, with a DRAM (Dynamic Random Access Memory). Accordingly, the degree of integration of memory cells within a single semiconductor chip is lower compared with that of the DRAM. Since the SRAM is basically composed of so-called flip-flop circuits, however, stored information is continuously maintained as long as a supply voltage is applied, thus requiring no refresh operation. It is therefore possible to simplify system configuration with the SRAM.
In the case of the SRAM, the capacitance of a storage node (accumulation node) has decreased along with the recent miniaturization of memory cells. Because of this decrease in capacitance, a so-called soft error problem has surfaced in which data retained at the storage node is inverted due to electrons generated by alpha rays released from a package, neutron rays flying in from the cosmic space, or the like. Hence, a variety of attempts have been made in order to reduce this soft error.
Japanese Patent Laid-Open Nos. 2004-241403, 2005-191454, 2006-140490 and 2006-245521 disclose configurations in which capacitance is provided between upper and lower electrodes, which are constituent elements of the SRAM, using a capacitance formation step dedicated to storage nodes.
In these configurations, however, a capacitance formation step is required in addition to usual steps of SRAM fabrication, thus incurring an increase in the number of manufacturing steps and in the cost of manufacture. Furthermore, these configurations involve forming a capacitor structure dedicated to a storage node and, therefore, unavoidably complicate device configuration. Consequently, it is difficult for these configurations to meet the recent requirement for further miniaturization with respect to the SRAM.